An aircraft comprises a large amount of equipment, of diverse nature, mechanical, hydraulic, electrical or electronic, whose proper operation is essential in the course of a flight.
To improve the degree of confidence accorded to this equipment, monitoring of its proper operation is undertaken, as often as possible, for each item. Monitoring generally comprises checking the fundamental parameters and automatic or semi-automatic tests of proper operation. Generally, a fault diagnosis is carried out and may lead to the emitting of fault messages.
A monitoring and alarm function makes it possible to detect a malfunction having an impact on the safety of the aircraft. A function of this type, also called “Flight Warning” in the art, is present on certain Airbus aircraft. Another function, generally, named “Flight Desk Effect” in the art, makes it possible to present these alarms to the crew.
Additionally, a maintenance function is associated with the monitoring function in order to diagnose faults and store them.
It is known by the name of BITE function, derived from the abbreviation of the expression “Built In Test Equipment”.
The BITE function of an item of equipment is taken on by electronics which may be specific or shared with other functions of the item of equipment considered. This electronics performs the software processing required by the BITE function.
It comprises a more or less significant hardware part secured to the item of equipment, with, at the minimum, in this hardware part, a nonvolatile memory. Certain data are stored in the volatile memory, including the breaching of standards by the monitored parameters, the results of the tests, the fault diagnostic when it exists as well as the fault messages emitted. The fault messages of the BITE functions of the monitored equipment of an aircraft are addressed, by an airplane data transmission link, to a centralizer item of equipment placed aboard the aircraft so as to bundle together the various fault messages emitted.
Aboard recent aircraft, fault messages originating from the BITE functions of the various items of equipment are consultable from the flight deck. They are furthermore preprocessed, with a view to easing the task of crews and maintenance personnel, by a specialized central computer known by various terms such as CMC from the expression “Central Maintenance Computer” or else CFDIU from the expression “Centralized Fault Display Interface Unit”.
This central maintenance computer is accessible by the crew through an interface with keyboard and screen which may be that known by the abbreviation MCDU derived from the expression “Multipurpose Control Display Unit” but which may also be a portable computer of the PC kind attached by a disconnectable data link which does or does not utilize the airplane bus.
Its main function is to effect, in real time or at the end of a flight, a diagnostic of the general situation of the aircraft. This diagnostic is carried out on the basis of a summary of the fault messages received from the various items of equipment of the aircraft.
It also fulfils other functions such as:                correlation of the fault messages received with the alarms received at the flight deck level;        running of particular tests on the items of equipment, conducted on request, by an operator intervening from the keyboard-screen interface giving access to the central maintenance computer;        drafting of a “post-flight” report, known by various terms such as PFR or LLR for “Post Flight Report” or “Last Leg Report”. This report is, generally, carried out for the maintenance teams on the ground.        
The latter function which consists in providing a “post-flight” report makes it possible to ease the work of the ground maintenance crew.
It comprises:                the log of the fault messages emitted by the various items of equipment of the aircraft;        the alarms presented to the crew;        the summary of the fault messages made as a last resort;        the information on the operating states of the items of equipment.        
The information featuring in this report results from automatic exploitation of the equipment fault messages or remarks by the crew.
The faults are generally correlated with the estimated lifetimes of the items of equipment. The lifetime of an item of equipment or of an electronic module results from a function which can:                either give the remaining lifetime, in this case it is generally called “time to live estimator” in the art, denoted TTL estimator;        or give a mean life time between two probable successive faults, in this case it is generally called “Mean Time Between Failure” in the art, customarily denoted MTBF.        
The MTBF is estimated, generally, in operating hours; it is estimated on the basis of a reliability model. It is given at the start of the life of the equipment or electronic module and is no longer readjusted or modified throughout the life cycle of the said equipment or electronic module.
The remaining lifetime, termed TTL, takes into account the possible degradations of an item of equipment and its MTBF.
These various monitoring, maintenance and diagnostic functions are implemented on equipment, modules or electronic cards or else links connecting these various components.
The entity that the maintenance operator can extract from a system so as to subject it to various tests during maintenance, repair it or replace it, is generally called an LRU, the terminology standing for “Line Replaceable Unit”,
Among these various functions, the reliability model used is theoretical and relies on probabilistic failure considerations. The MTBF of an LRU is currently determined at the start of the life cycle of the said LRU.
This MTBF data item for each LRU makes it possible to table a maintenance plan for each LRU included within an overall maintenance plan.
One of the main concerns is to schedule a maintenance and replacement plan for the various LRUs representing the actual fault frequencies, the aim being to immobilize an LRU for maintenance operations, neither too early nor too late.
FIG. 1 represents the curve representing the standard reliability of an electronic item of equipment, such as those which are integrated within aircraft. The shape of the curve is typical and is called a “bathtub” curve.
The axis 1 represents the failure rate of the LRU, the axis 2 represents the time axis along which the life cycle of the LRU proceeds.
A first slope 3 represents the failure rate of an item of equipment, at the very start of its life. Defective items of equipment are, generally, eliminated after the first tests.
The part of the curve 4 represents a nearly constant failure rate of the equipment. This portion represents the mean time between two faults. Then the slope 5 represents an increasing failure rate relating to the end of the life of the equipment. In practice, equipment rarely goes beyond the part represented by the slope 5.
A major drawback in equipment maintenance, in general, is not to take account of events, throughout the life cycle of the said equipment, in the diagnostics. Therefore, usage is often poorly optimized and results from the errors in assessment between on the one hand the actual lifetime of the equipment and on the other hand the theoretical lifetime.
Additionally, the costs generated by immobilizing certain hardware and the false fault rates are drawbacks which make it necessary to readjust the maintenance data, and more particularly the MTBF data.
The major drawback of this reliability model resides in the fact that events cropping up in the course of the life cycle, such as a change of hardware configuration of the LRU, are not taken into account during the evaluation of the MTBF, the latter being determined initially.
Additionally, today the aeronautical industry has feedback of experience regarding the reliability models of the LRUs deployed on aircraft. It is possible to analyse, for example, the actually noted values of the lifetimes of the LRUs and theoretically estimated lifetimes.
On the other hand, there is no existing means of integrating this feedback of experience into the centralized maintenance system.